A better solution was found and has been pushed to the main repository.

In short, the inclusion of the `bootstrapped/lib' directory was mandated to all Makefiles from `configure.ac'. So to fix the problem, this directory is now only included when building the libraries and utilities (with `AM_CPPFLAGS' in the separate `Makefile.am's), not when building the tests.

As a sidenote: due to a bootstrapping update, the library development branch was merged with master before this commit. However, the library development will again continue on that separate branch.

As part of task #13786, a test was written to compile a simple C++ program, it worked on my GNU/Linux system, but a friend of mine who tested it on a Mac came up with the following errors when running `make check'.

The error is apparently due to the fact that his system is looking into the bootstrapped Gnulib C headers (for example `../bootstrapped/lib/string.h' in the first case), when it is compiling the C++ `versioncpp.cpp' code.

When building actual Gnuastro code, we need to include the Gnulib libraries before the system libraries, so we have the `../bootstrapped/lib' directories in `CPPFLAGS'. However, this is not the case for tests, we want the tests to completely use the system's resources (since we won't be installing Gnulib).

By explicitly setting `CPPFLAGS' in `tests/Makefile.am' as shown below, the problem on his system was solved and it successfully built `versioncpp'.

But this can't be a permanent solution, since we are practically over-writing this important user-defined variable.